- have best corrected visual acuity of 6/12 to approximately 6/96 (Snellen equivalent), assessed with the use of charts from the Early Treatment Diabetic Retinopathy Study (ETDRS) (70 to 25 ETDRS 1 m equivalent letter scores; patients initially view the charts at a starting distance of 4 m, the number of correctly read letters are given a correction factor with the final letter score being the equivalent of a patient reading it at 1m. A score of 55 letters approximates to 6/24 Snellen acuity)